Solar Cells Based on Colloidal Nanocrystals

236 Pages·2014·5.939 MB·English

by  Holger Borchert (auth.)

Springer Series in Materials Science 196
Holger Borchert
Solar Cells
Based on
Colloidal
Nanocrystals
Springer Series in Materials Science
Volume 196
Series editors
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Chennupati Jagadish, Canberra, ACT, Australia
Richard M. Osgood, New York, USA
Jürgen Parisi, Oldenburg, Germany
Shin-ichi Uchida, Tokyo, Japan
Zhiming M. Wang, Chengdu, People’s Republic of China
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future device technologies, the book titles in this series reflect the state-of-the-art
in understanding and controlling the structure and properties of all important
classes of materials.
Holger Borchert
Solar Cells Based
on Colloidal Nanocrystals
123
Holger Borchert
Department of Physics
Carl-von-Ossietzky University
ofOldenburg
Oldenburg
Germany
ISSN 0933-033X ISSN 2196-2812 (electronic)
ISBN 978-3-319-04387-6 ISBN 978-3-319-04388-3 (eBook)
DOI 10.1007/978-3-319-04388-3
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Preface
Solar cells involving colloidal nanocrystals are a rapidly developing field of
research. Many physical and chemical properties of crystalline solids can signif-
icantly change when the spatial dimensions of the crystallites are reduced to the
nanometersizeregime.Thisopenspossibilitiestotunematerialpropertiesinview
of specific applications. With respect to thin film photovoltaics, semiconductor
nanocrystals have the potential to be used as tunable materials for efficient
absorption of sunlight, either in combination with conductive polymer or also in
inorganic absorber layers. Thereby, chemical approaches to synthesize the nano-
particlesinliquidmediagiverisetothepossibilityofproducingabsorberlayersby
deposition of the materials from solution. Therefore, similar as in the field of
organic photovoltaics, relatively simple and cost-efficient processes like printing
technologies may be used for the realization of corresponding thin films.
Inthecaseoforganicphotovoltaicswhichitselfisacomparablyyoungandstill
developing field, several books have appeared in recent years, giving good over-
views and deep insight into that technology. Approaches to combine conductive
polymer with inorganic semiconductor nanocrystals in hybrid systems are some-
times treated as a side-aspect in books on organic photovoltaics or organic elec-
tronics, but the literature specialized particularly on solar cells with inorganic
nanocrystals is still rare. On the other hand, nanoparticle-based solar cells have
made an impressive development in recent years, have their own particularities,
and should merit more attention in terms of books focusing particularly on them.
This was the main source for my motivation to write the present book.
Researchonsolarcellswithcolloidalnanoparticlesisstronglyinterdisciplinary
and covers many aspects of physics, chemistry, and materials science. The book
aims at bridging gaps between the involved scientific disciplines andcollectsinto
one work important fundamentals from different fields. The book reflects the
current state of research on relevant materials and different types of nanoparticle-
based solar cells. It addresses researchers, Ph.D. students, engineers, and others
interestedintheapplicationofcolloidalnanoparticlesinphotovoltaics.Moreover,
the book may also serve as an advanced textbook to accompany specialized lec-
tures in physics, chemistry, materials science, and related areas.
The book is organized into three parts, the first of them addressing specific
propertiesofcolloidalnanocrystalsaswellasconductivepolymeringeneral.The
secondpartfocusesonaselectionofcharacterizationmethodsrelevantforthefield.
v
vi Preface
Thereby, short introductions to the different methods are given, and their appli-
cation potential for exploring the properties of materials and solar cells is
discussed.Thethirdpartofthebookdescribesdifferentconceptsforusingcolloidal
nanocrystalsinsolarcellsandreviewsthestateoftheartandrecentdevelopments
and tendencies in this research area.
As the author, I would like to express my gratitude to all who supported the
writingofthebook,eitherbyreadingpartsofthemanuscriptorhelpingmeinthe
planning of the book. Namely, I would like to mention here my wife, Dr. Yulia
Borchert, as well as my present, respectively, former colleagues Dr. Martin
Knipper, Dr. Marta Kruszynska, Dr. Florian Witt, and Prof. Dr. Elizabeth von
Hauff.IamalsoparticularlygratefultoProf.Dr.JürgenParisiforhisadviceinthe
planning and in whose working group I got the opportunity to perform active
research in the scientific field which the present book is focused on. I hope to
provide with this book a useful and appealing work and hope the readers will
enjoy it.
Oldenburg, February 2014 Holger Borchert
Contents
1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Part I Materials
2 Physics and Chemistry of Colloidal Semiconductor Nanocrystals . 15
2.1 Basic Concepts of Colloidal Synthesis . . . . . . . . . . . . . . . . . 15
2.2 Short Overview on Materials. . . . . . . . . . . . . . . . . . . . . . . . 19
2.3 Material Properties Depending on Particle Size . . . . . . . . . . . 20
2.4 Material Properties Related to the Surface of Colloidal
Nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3 Physics and Chemistry of Conductive Polymers. . . . . . . . . . . . . . 39
3.1 Electrical Conductivity in Organic Materials . . . . . . . . . . . . . 39
3.1.1 Hybridization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.1.2 Conjugated Double Bonds. . . . . . . . . . . . . . . . . . . . 42
3.1.3 The Structure and Conductivity
of Trans-Polyacetylene . . . . . . . . . . . . . . . . . . . . . . 45
3.2 Different Types of Conductive Polymer . . . . . . . . . . . . . . . . 51
3.3 Physical and Chemical Properties of Conductive Polymer. . . . 54
3.3.1 Structural Properties: Chain Length
and Regioregularity . . . . . . . . . . . . . . . . . . . . . . . . 54
3.3.2 Absorption Properties . . . . . . . . . . . . . . . . . . . . . . . 57
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
vii
viii Contents
Part II Characterization of Colloidal Nanocrystals
and Thin Polymer Films
4 Electron Microscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.1 Basics of Electron Microscopy. . . . . . . . . . . . . . . . . . . . . . . 63
4.2 High-Resolution Transmission Electron Microscopy
(HRTEM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.3 Fourier Analysis and Image Filtering . . . . . . . . . . . . . . . . . . 69
4.4 Particle Size Determination . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.5 Sample Preparation and Stability . . . . . . . . . . . . . . . . . . . . . 73
4.6 Scanning Electron Microscopy (SEM). . . . . . . . . . . . . . . . . . 74
4.7 Electron Tomography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
5 X-ray Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.1 Basics of X-ray Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.2 Particle Size Determination . . . . . . . . . . . . . . . . . . . . . . . . . 86
5.3 Rietveld Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
5.4 Small Angle X-ray Scattering (SAXS) . . . . . . . . . . . . . . . . . 91
5.5 X-ray Diffraction of Soft Matter. . . . . . . . . . . . . . . . . . . . . . 92
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
6 Photoelectron Spectroscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
6.1 Fundamentals of X-ray Photoelectron Spectroscopy . . . . . . . . 95
6.2 Surface Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.3 High-Resolution Photoelectron Spectroscopy
of Semiconductor Nanocrystals . . . . . . . . . . . . . . . . . . . . . . 100
6.4 Quantitative Photoelectron Spectroscopy: Depth Profiles
of the Chemical Composition. . . . . . . . . . . . . . . . . . . . . . . . 104
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
7 Cyclic Voltammetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
7.1 Fundamentals of Cyclic Voltammetry. . . . . . . . . . . . . . . . . . 111
7.2 Examples for the Study of Energy Levels in Organic
Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
7.3 Analysis of Defect States in Colloidal Semiconductor
Nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
8 Absorption and Photoluminescence Spectroscopy. . . . . . . . . . . . . 119
8.1 Fundamentals of Absorption Spectroscopy. . . . . . . . . . . . . . . 119
8.2 Fundamentals of Photoluminescence Spectroscopy. . . . . . . . . 121
8.3 Photoinduced Absorption Spectroscopy. . . . . . . . . . . . . . . . . 123
8.4 Time-Resolved Optical Spectroscopy . . . . . . . . . . . . . . . . . . 125
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
Contents ix
9 Electron Spin Resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
9.1 Fundamentals of Electron Spin Resonance Spectroscopy. . . . . 129
9.2 Light-Induced Electron Spin Resonance (L-ESR)
Spectroscopy as a Probe for Charge Transfer Processes
in Donor/Acceptor Systems . . . . . . . . . . . . . . . . . . . . . . . . . 132
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
10 Electrical Characterization of Solar Cells . . . . . . . . . . . . . . . . . . 139
10.1 Current–Voltage Measurements . . . . . . . . . . . . . . . . . . . . . . 139
10.1.1 Fundamentals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
10.1.2 Measurement Conditions. . . . . . . . . . . . . . . . . . . . . 142
10.2 Quantum Efficiency Measurements. . . . . . . . . . . . . . . . . . . . 146
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
11 Charge Carrier Mobility Measurements . . . . . . . . . . . . . . . . . . . 149
11.1 General Aspects of Charge Transport . . . . . . . . . . . . . . . . . . 149
11.2 Organic Field Effect Transistors. . . . . . . . . . . . . . . . . . . . . . 151
11.3 Single Carrier Diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
Part III Solar Cells with Colloidal Nanocrystals
12 Hybrid Polymer/Nanocrystal Solar Cells. . . . . . . . . . . . . . . . . . . 159
12.1 Potential Advantages of Using Inorganic Nanocrystals
as Alternative Electron Acceptors. . . . . . . . . . . . . . . . . . . . . 159
12.2 Material Combinations for Hybrid Solar Cells . . . . . . . . . . . . 162
12.2.1 Solar Cells Based on Cadmium Chalcogenides . . . . . 162
12.2.2 Solar Cells Based on Lead Chalcogenides. . . . . . . . . 169
12.2.3 Solar Cells Based on Ternary I–III–VI
Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
12.2.4 Solar Cells Based on III–V Semiconductors. . . . . . . . 176
12.2.5 Solar Cells Based on Transition Metal Oxides. . . . . . 176
12.2.6 Solar Cells Based on Silicon Nanocrystals. . . . . . . . . 179
12.3 Elementary Processes in Hybrid Solar Cells and Strategies
for Improvement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
12.3.1 Charge Separation at the Organic–Inorganic
Donor–Acceptor Interface . . . . . . . . . . . . . . . . . . . . 180
12.3.2 Charge Transport in Organic–Inorganic
Hybrid Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
12.3.3 Defects and Charge Carrier Trapping in Hybrid
Solar Cells. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
12.3.4 Alternatives to Ligand Exchange as Requirement
for Hybrid BHJ Solar Cells . . . . . . . . . . . . . . . . . . . 195
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197